1. Field of the Invention
The present invention relates to a multilayer resin wiring board including a metal core substrate, wiring layers, and intervening insulating resin layers, and to a method for manufacturing the same. More particularly, the invention relates to a multilaye resin wiring board including a metal core substrate having a through hole formed thereir and to a method for manufacturing the same.
2. Description of the Related Art
Multilayer resin wiring boards are generally known to include alternately-arrranged insulating resin layers and wiring layers and can further include a single or a plurality of metal core substrates. For example, the prior art describes multilayer resin wiring board that include a metal core substrate, insulating resin layers and wiring layers which are alternately arranged on upper and lower sides of the metal core substrate, wherein the metal core substrate of such wiring boards is brought to the grounding potential or a power source potential.
When such a metal core substrate is used and is to be electrically connected to two wiring layers located on its opposite sides, a through hole is drilled in the metal cor substrate and insulating resin layers located on opposite sides of the metal core substrate. A via connected to the metal core substrate is formed on the wall of the through hole, thereby establishing electrical connection among the wiring layers and the metal core substrate.
In order to meet demand for fine patterns, compactness, and high degree of integration of printed wiring to be formed on a resin wiring board, the diameter of through holes has been decreased. When through holes are formed by drilling, however, a long period of time is required. The long drilling time coupled with the very small diameter of the through holes results in an increased possibility of breakage of the drill. Particularly, when the diameter of a through hole is to be less than 200 .mu.m (0.2 mm), drilling becomes significantly difficult. In addition, wear to the cutting edge requires that the drill be replaced after a predetermined number of holes are drilled or total drilling time reaches a predetermined length.
To avoid the above-mentioned problems, the inventors have proposed machining of through holes. In particular, the inventors propose using an ultraviolet laser such as the fourth harmonic of a YAG laser to bore a through hole in insulating resin layers and a metal core substrate, such as a copper core substrate. Laser machining can easily bore finer through holes, e.g., 50 .mu.m or smaller, than drilling. Further, laser machining does not result in breakage or replacement of drills and can improve boring speed and accuracy of boring position, thereby enabling micromachining.
In the case of a multilayer resin wiring board (hereinafter simply referred to as a PWB) insulating layers of a resin or a resin-containing composite material and thin (10 .mu.m to 20 .mu.m) wiring layers are alternately arranged on a thin (for example, 20 .mu.m or 35 .mu.m thick) metal core substrate. However, rigidity is sacrificed when using such thin layers and could result in bending or deformation of the PWB. Consequently, when electronic devices (for example, integrated-circuit chips) are mounted on the PWB, stress is applied to a connection between the PWB and an electronic device due to deformation of the PWB, potentially resulting in breakage of the connected portion.
In order to prevent this problem, a relatively thick (not less than 50 .mu.m thick; e.g., 100 .mu.m or 200 .mu.m thick) metal core substrate may be used in an attempt to improve the rigidity of the entire PWB. Unlike a thin metal core substrate which is likely to deform, wrinkle or fold and is thus difficult to handle, a thick metal core substrate is easy to handle and hardly wrinkles, which is an advantageous during processing. However, it is difficult to bore through holes in a thick metal core substrate therein by laser machining. Namely, the time required for boring a through hole in a thick metal core substrate becomes excessively long, resulting in a significant increase in machining man-hours. Further, heat generated during laser machining causes an increase ir resin smear, further resulting in an inability to accurately bore a fine through hole, which ultimately causes deterioration of the insulating resin layer at a portion around the bored through hole.